Drill bit with staged durability, stability and rop characteristics

ABSTRACT

Drill bit with multiple stages of durability and ROP characteristics is disclosed. The drill bit has multiple layers of cutters established by deploying the cutters on blades of different heights or maintaining the blades at the same height and deploying the cutters to have different heights on one or more blades. Each layer provides independent bottom hole coverage and has independent stabilization, ROP, and durability characteristics so as to effectively drill through different subsurface formations. Cutters deployed on the different layers have their respective centers at substantially different radial positions. Due to the different radial positions, cutters in different layers cut different swaths in the subsurface formation. Cutters in different layers may also have different initial peripheral portions or shear lengths, resulting in different impact resistance characteristics for the different layers. This changes the wear and/or cutter deterioration processes for the different layers, resulting in different and/or improved toughness characteristics.

CROSS REFERENCE TO RELATED APPLICATION

This application for patent claims priority to, and hereby incorporatesby reference, U.S. Provisional Application Ser. No. 60/746767, entitled“Drill Bit with Staged Durability and ROP Characteristics,” filed May 8,2006 with the United States Patent and Trademark Office.

FIELD OF THE INVENTION

The present invention relates to rotary drill bits for rotary drillingof subterranean formations and, more specifically, to a rotary drill bithaving multiple stages of durability and ROP (rate of penetration)characteristics.

BACKGROUND OF THE INVENTION

Subsurface formation drilling to recover hydrocarbons is well known inthe art. The equipment for such subsurface formation drilling typicallycomprises a drill string having a rotary drill bit attached thereto thatis lowered into a borehole. A rotary table or similar device rotates thedrill string, resulting in a corresponding rotation of the drill bit.The rotation advances the drill bit downwardly, causing it to cutthrough the subsurface formation (e.g., by abrasion, fracturing, and/orshearing action). Drilling fluid is pumped down a channel in the drillstring and out the drill bit to cool the bit and flush away any debristhat may have accumulated. The drilling fluid travels back up theborehole through an annulus formed between the drill string and theborehole.

Many types of drill bits have been developed, including roller conebits, fixed cutter bits (“drag bits”), and the like. For each type ofdrill bit, several patterns or layouts of cutting elements (“cutters”)are possible, including spiral patterns, straight radial patterns, andthe like. Different types of cutting elements have also been developed,including milled cutting elements, tungsten carbide inserts (“TCI”),polycrystalline-diamond compacts (“PDC”), and natural diamond cuttingelements. The selection of which particular drill bit, cutting elementtype, and cutting element pattern (i.e., cutting structure) to use for agiven subsurface formation can depend on a number of factors. Forexample, certain combinations of drill bit, cutting element type, andcutting element pattern drill more efficiently and effectively in hardformations than others. Another factor is the range of hardnessencountered when drilling through the different formation layers.

One common pattern for drill bit cutting elements in a fixed cutterdrill bit is a spiral configuration an example of which is shown inFIGS. 1A-1B. As can be seen, a spiral pattern drill bit 100 is composedof several sections, including a bit body 102, a shank 104, and athreaded connector 106 for connecting the drill bit 100 to a drillstring. Flats 108 on the shank 104 allow a tool, such as wrench to gripthe drill bit 100, making it possible (or at least easier) to screw thedrill bit 100 onto the drill string. Blades 110 a, 110 b, 110 c, 110 d,110 e, and 110 f are formed on the drill bit 100 for holding a pluralityof cutting elements 112. The cutting elements 112 include superabrasivefaces that usually have identical geometries (i.e., size, shape, andorientation), but different positions and/or cutting angles (back rakeangles) on the blades 110 a-f Also visible are drill fluid outlets 114that conduct the drilling fluid away from the drill bit 100, therebyremoving any debris and cuttings that may have accumulated.

With existing drill bit configurations, it is known to have blades 110a-f and/or cutting elements 112 that are offset (i.e., have differentheights) relative to other blades and/or cutting elements on the drillbit 100. The height of the blades 110 a-f and/or cutting elements 112 ismeasured herein relative to the drill bit body 102. For blades 110 a-fthat are offset, the cutting element tips are set at the same heightrelative to each other, but one or more of the blades 110 a-f have aheight that is greater than one or more other blades 110 a-f. Where thecutting elements 112 are offset, the blades 110 a-f have the sameheight, but the tips of certain cutting elements are set at differentheights relative to other cutting elements. In either case, the endresult is a primary layer of cutting elements that performs the initialdrilling, followed by a secondary layer of cutting elements, and in someapplications, a tertiary layer and so forth as needed.

Because of the difference in height, the primary layer of cuttingelements wears away or deteriorates faster than the secondary layer ofcutting elements. As the primary cutting elements progressively wearaway, however, the secondary cutting elements compensate increasinglymore for the decreased effectiveness of the primary cutting elements interms of bit durability and ROP. This allows the drill bit 100 to beable to drill at an acceptable ROP for longer durations before having tobe replaced, in essence performing the work of multiple (e.g., two inthis instance) drill bits 100.

The above arrangements are illustrated in FIGS. 2A-2B, where a segmentof a drill bit profile 200 for a drill bit having multiple layers ofcutting elements is shown. The term “profile,” as understood by thosehaving ordinary skill in the art, refers to the area outlined by thecutting elements when rotated onto the same radial plane. As can beseen, the drill bit profile segment 200 includes two layers of cuttingelements, a primary layer 202 and a secondary layer 204, that are offsetfrom each other. The offset in the layers of cutting elements 202 and204 is indicated here by the letter H. Each layer 202 and 204 iscomposed of a plurality of individual cutting elements 206 a-n and 208a-n, respectively. For clarity purposes, the layers of cutting elements202 and 204 shown here and in the remaining figures are depicted asrelatively flat. Those having ordinary skill in the art will recognizethat, in practice, the layers of cutting elements 202 and 204 may have adegree of curvature.

In existing drill bits, the cutting elements are essentially uniform insize and shape (typically a round shape). In addition, the cuttingelements 206 a-n of the primary layer 202 and the cutting elements 208a-n of the secondary layer 204 share substantially the same radialpositions on their respective blades, or share a common reference axis.FIG. 2B illustrates an example of such radial position sharing where,for clarity purposes, only one cutting element from each layer 202 and204 is shown. As can be seen, the centers C of the primary layer cuttingelements (e.g., cutting element 206 a) and the centers C′ of thesecondary layer cutting elements (e.g., cutting element 208 a)substantially line up on a common radial position (see radialdisplacement X) when rotated onto the same radial plane.

Furthermore, the cutting elements 206 a-n of the primary layer 202 andthe cutting elements 208 a-n of the secondary layer 204 havesubstantially the same shear length (SL), shown in FIG. 2A by the heavyarcuate line. “Shear length” refers to the portion of a cuttingelement's periphery that is in direct contact with the formation beingdrilled when all cutting elements on the drill bit (and particularly inthe vicinity of the cutting element being measured) are rotated onto asingle radial plane. A cutting element's shear length is typicallymeasured while the drill bit is new (i.e., unused) and depends in largepart on the size of the cutting element and adjacent cutting elements,although it is possible to vary the shear length for different drillbits even when the cutting elements are all of the same size.

The shear length affects the ability of the drill bit to penetratevarious types of formation material. For example, hard and abrasiveformations requiring a high level of bit stabilization are moreeffectively drilled with drill bits having longer shear lengths. Softformation materials, on the other hand, cause minimal impact damage andmay therefore be effectively drilled with either longer or shorter shearlengths. Shear length also affects the level of stabilization needed tominimize impact damage, thus reducing the amount of cutting elementdeterioration For existing drill bits with the type of dual-layerprofile shown in FIG. 2A, the cutting elements in the primary andsecondary layers have identical shear lengths.

Because of the identical shear lengths, and also because of the sharedradial positions, the cutting elements 206 a-n and 208 a-n in theprimary and secondary layers 202 and 204 cut identical swaths throughthe subsurface formation. This is illustrated in FIGS. 3A-3B, wheredifferent implementations of drill bits having a dual-layer profile areshown.

Referring first to FIG. 3A, a dual-layer drill bit profile may beachieved by providing a drill bit 300 where the blades 302 a and 302 bhave different heights. In the portion shown here, the blade in thelower half of the figure, blade 302 a, has a greater height than theother blade 302 b. Therefore, the cutting elements 306 a-n on the firstblade 302 a constitute the primary layer of cutting elements, while thecutting elements 306 a-n on the second blade 302 b constitute thesecondary layer of cutting elements.

Turning now to FIG. 3B, a dual-layer drill bit profile may also beachieved by providing a drill bit 310 where the blades 312 a and 312 bhave the same heights, but the cutting elements 314 a-n and 316 a-n areset at different heights relative to each other. In the portion shownhere, the non-shaded cutting elements 314 a-n are set at a greaterheight than the shaded cutting elements 316 a-n. These non-shadedcutting elements 314 a-n are therefore part of the primary layer ofcutting elements, whereas the shaded cutting elements 316 a-n constitutepart of the secondary layer of cutting elements. Both the shaded andnon-shaded cutting elements 314 a-n and 316 a-n may be intermixed acrossthe blades 312 a and 312 b, as depicted here, or cutting elements ofdifferent heights may be mounted on different blades, respectively(similar to the implementation of FIG. 3A).

Because of the same shear lengths and common radial positions, primarylayer and secondary layer cutting elements at a given radial positionnecessarily cut the same swath (see dashed lines) in the subsurfaceformation This is the case regardless of the specific deployment ofcutting elements used to achieve the primary layer and secondary layers.The width of the swath or “cutting zone” created by cutting elements ondifferent blades sharing a common radial position is indicated here bythe letter Z and is equal to the diameter D of the cutting elements.Because they cut the same swath the primary layer and secondary layer donot establish independent coverage of the bottom hole. In addition, andfrom a geometry standpoint and also based on their shear lengths, thewear and/or deterioration process on the cutting element typicallystarts from the same peripheral locations on the cutting elements forcutting elements in the different layers. This arrangement has anegative effect on overall bit performance, especially durability orlongevity.

Thus, despite certain advances made in the industry, there remains aneed for a drill bit having an improved cutting element arrangement thatenhances stabilization as well as durability and ROP characteristics,and permits the drill bit to drill at economical ROPs for longerdurations and through a wider range of formation materials withouthaving to replace the drill bit, thereby reducing costly andtime-consuming bit trips.

SUMMARY OF THE INVENTION

Embodiments of the invention are directed to a drill bit, and method ofassembling same, that can drill at economical ROPs for longer durationsand in a wider range of formation materials. The drill bit has multiplelayers of cutting elements established by deploying the cutting elementson blades of different heights or maintaining the blades at the sameheight and deploying the cutting elements to have different heights onone or more blades. Each layer provides independent bottom hole coverageand has independent stabilization, ROP, and durability characteristicsso as to effectively drill through different subsurface formations.Cutting elements deployed in different layers have their respectivecenters at substantially different radial positions. Due to thedifferent radial positions, cutting elements in different layers cutdifferent respective swaths in the subsurface formation, and are thusloaded and deteriorate differently and independently of the otherlayers. Cutting elements in different layers may also have differentinitial peripheral portions or shear lengths, resulting in differentimpact resistance characteristics for the different layers. Thisdrastically changes the wear and/or cutting element deteriorationprocesses for the different layers, which results in different andimproved toughness characteristics. In some embodiments, cuttingelements deployed on different layers have different sizes, shapes,and/or back rake angles, respectively. In other embodiments, cuttingelements deployed on different layers have different thermal stability,impact resistance, and/or abrasion resistance, respectively.

In general, in one aspect, the invention is directed to a drill bit. Thedrill bit comprises a drill bit body, blades formed on said drill bitbody, said blades having a plurality of cutting element positionsradially located thereon. The drill bit further comprises cuttingelements deployed on said blades, said cutting elements forming aprimary layer of cutting elements and a secondary layer of cuttingelements, said primary layer of cutting elements having a differentheight relative to said drill bit body from said secondary layer ofcutting elements. At least one primary layer cutting element and acorresponding secondary layer cutting element occupy substantiallydifferent radial cutting element positions on said blades such thattheir cutting element profiles overlap, said at least one primary layercutting element and said corresponding secondary layer cutting elementtogether defining a cutting zone equal to a diameter of one of said atleast one primary layer cutting element and said corresponding secondarylayer cutting element plus a predetermined percentage or fraction ofsaid diameter.

In general, in another aspect, the invention is directed to a method ofassembling a drill bit. The method comprises providing a drill bit bodyhaving blades formed thereon, said blades having a plurality of cuttingelement positions radially located thereon. The method further comprisesdeploying cutting elements on said blades, said cutting elements forminga primary layer of cutting elements and a secondary layer of cuttingelements, said primary layer of cutting elements having a differentheight relative to said drill bit body from said secondary layer ofcutting elements. At least one primary layer cutting element and acorresponding secondary layer cutting element occupy substantiallydifferent cutting element positions on said blades such that theircutting element profiles overlap, said at least one primary layercutting element and said corresponding secondary layer cutting elementtogether defining a cutting zone equal to a diameter of one of said atleast one primary layer cutting element and said corresponding secondarylayer cutting element plus a predetermined percentage or fraction ofsaid diameter.

In general, in yet another aspect, the invention is directed to a drillbit body. The drill bit body comprises blades formed on said drill bitbody and cutting element positions formed on said blades. The cuttingelement positions are radially located such that when cutting elementsare deployed on said blades, said cutting elements form a primary layerof cutting elements and a secondary layer of cutting elements, saidprimary layer of cutting elements having a different height relative tosaid drill bit body from said secondary layer of cutting elements. Atleast one primary layer cutting element and a corresponding secondarylayer cutting element occupy substantially different cutting elementpositions on said blades such that their cutting element profilesoverlap when said cutting elements are deployed on said blades, said atleast one primary layer cutting element and said corresponding secondarylayer cutting element together defining a cutting zone equal to adiameter of one of said at least one primary layer cutting element andsaid corresponding secondary layer cutting element plus a predeterminedpercentage or fraction of said diameter.

In general, in another aspect, the invention is directed to a drill bitcapable of drilling effectively in long intervals of formation materialor sections having grossly different mechanical and/or geologicproperties (i.e. sandstone, carbonates and chert or pyrite).

In general, in still another aspect, the invention is directed to adrill bit capable of effectively drilling in formations infested withchert, pyrite or nodules, where these specific materials are located atthe top, middle or bottom sections of the formation interval, and whereconventional drilling practices typically require the use of multipledrill bits, which may have drastic effects on drilling and operationalcosts.

Additional aspects of the invention will be apparent to those ofordinary skill in the art in view of the detailed description of variousembodiments, which is made with reference to the drawings, a briefdescription of which is provided below.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages of the invention will become apparentfrom the following detailed description and upon reference to thedrawings, wherein:

FIGS. 1A-1B, described previously, illustrate a side view and a bottomview of a prior art fixed cutting element drill bit;

FIGS. 2A-2B, described previously, illustrate a segment of a drill bitprofile for a drill bit having multiple cutting element tip heightswhere cutting elements with different tip heights occupy substantiallythe same radial positions and have substantially the same size, shape,and shear length;

FIGS. 3A-3B, described previously, illustrate blades for drill bitshaving the profile shown in FIGS. 2A-2B;

FIGS. 4A-4B illustrate a segment of a drill bit profile for a drill bithaving multiple layers of cutting elements where cutting elements withdifferent tip heights occupy substantially different radial positions;

FIGS. 5A-5B illustrate exemplary implementations of a drill bit havingthe drill bit profile shown in FIGS. 4A-4B;

FIGS. 6A-6B illustrate alternative implementations for a drill bithaving the drill bit profile shown in FIGS. 4A-4B;

FIGS. 7A-7B illustrate a segment of a drill bit profile for a drill bithaving multiple layers of cutting elements where cutting elements withdifferent tip heights occupy substantially different radial positionsand have substantially different shear lengths;

FIGS. 8A-8B illustrate a segment of a drill bit profile for a drill bithaving multiple layers of cutting elements where cutting elements withdifferent tip heights occupy substantially different radial positionsand have substantially different diameters;

FIGS. 9A-9B illustrate a segment of a drill bit profile for a drill bithaving multiple layers of cutting elements where cutting elements withdifferent tip heights occupy substantially different radial positionsand have substantially different axial volumes;

FIGS. 10A-10C illustrate a segment of a drill bit profile for a drillbit having multiple layers of cutting elements where cutting elementswith different tip heights occupy substantially different radialpositions and have substantially different back rake angles; and

FIGS. 11A-11B illustrate a segment of a drill bit profile for a drillbit having multiple layers of cutting elements where cutting elementswith different tip heights occupy substantially the same radialpositions and have substantially different shear lengths.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS OF THE INVENTION

Following is a detailed description of the invention with reference tothe drawings. It should be noted that the drawings are provided forillustrative purposes only and are not intended to be a blueprint ormanufacturing drawings, nor are they drawn to any particular scale.

As mentioned above, existing drill bits have primary layer and secondarylayer cutting elements that are uniform in size and shape and share thesame radial positions. This results in the cutting elements of thesecondary layers following an identical swath through the subsurfaceformation as the cutting elements of the primary layers. Consequently,the primary layer and secondary layer cutting elements do not establishindependent coverage of the bottom hole. In this regard, the tips of thecutting elements in the different layers, which are the first points ofcontact with the formation being drilled, begin to fail or deterioratealong a common line. Such failure initiation at the tips of the cuttingelements in the primary layer immediately exposes the cutting elementsin the secondary or other layers to failure also because the mentionedtips are in a straight line. As such, the secondary or other layer'sfailure is dependent on the initiation and rate of failure of thesecondary or other layer. This arrangement or deployment in existingdrill bits has a negative effect on bit performance. In addition, thecutting elements of the primary layers and the cutting elements of thesecondary layers have substantially the same shear lengths. Thisdecreases the effectiveness of existing drill bits, in terms of theirdurability, stabilization and ROP, thus narrowing the range of formationmaterials with which they can be used.

Embodiments of the invention provide a drill bit where at least onecutting element of the primary layer occupies a substantially differentradial position from a corresponding cutting element of the secondarylayer. Specifically, the at least one primary layer cutting element andthe corresponding secondary layer cutting element together define acutting zone that spans a predetermined distance. In some embodiments,the predetermined distance may be the diameter of one of the cuttingelements plus a certain percentage or fraction of that diameter. Thediameter may be that of the primary layer cutting element or it may bethat of the secondary layer cutting element, and the fraction may beabout 1/10 to about ⅓ of that diameter. As a result, the differentlayers establish bottom hole coverages that are independent of eachother. In one embodiment, the primary layer establishes nearly 100%coverage of the bottom hole and the secondary layer establishes at least80% coverage. This determination is typically based on the intendedapplication and may be influenced by bit size, blade count, bit profile,as well as the lengths of the secondary and tertiary blades used in thebit design.

The cutting elements of the different layers having different cuttingelement tip heights are deployed so as to define different layouts orcutting structures. In one embodiment, the different layers are adaptedfor specific applications based on the ROP, durability, andstabilization requirements of different drilling environments. Thecutting elements may be any suitable type of cutting element known tothose having ordinary skill in the art, including TCI cutting elements,PDC cutting elements, natural diamond cutting elements, and combinationsthereof. Furthermore, cutting elements deployed on the primary layer andcutting elements deployed on the secondary layer may have substantiallydifferent shear lengths, sizes, shapes, back rake angles, thermalstability, abrasion resistance and/or impact resistance. Based on theblade count and cutting element type and deployment, the drill bit ofthe invention may be customized for specific subsurface formations,including formations infested with chert, nodules, and/or pyrite. Thedrill bit of the invention is capable of drilling through formationswith such infestations at acceptable ROPs regardless of the location ofthe chert, nodules, and/or pyrite in a given formation interval or holesection Such an arrangement results in a drill bit that can drill ateconomical ROPs for longer durations and with a wider formationbandwidth, thus reducing costly and time-consuming bit trips. Note thatwhile embodiments of the invention are described herein mainly withrespect to drill bits having a primary layer and a secondary layer ofcutting elements, the teachings and principles discussed are fullyapplicable to drill bits having one or more additional layers of cuttingelements (e.g., a tertiary layer, etc.).

Referring now to FIGS. 4A-4B, a segment of a drill bit profile 400 for adrill bit having multiple layers of cutting elements, achieved bydeploying the cutting elements on blades of different heights ormaintaining the blades at the same height, then deploying the cuttingelements so as to have different heights on one or more blades, isshown. As can be seen in FIG. 4A, the drill bit profile segment 400includes a primary layer 402 that is offset in height relative to asecondary layer 404. Note that the two layers 402 and 404 are shown herein isolation (i.e., the amount of offset is exaggerated) so that thecutting elements 406 a-n and 408 a-n may be more easily seen. Thosehaving ordinary skill in the art will understand that, in practice, theoffset is much smaller than illustrated and more closely resembles theoffset shown in FIGS. 2A-2B. It should also be noted that the number oflayers of cutting elements is not necessarily limited to two. In someinstances, depending on the drilling application and the challengespresented, there may be a primary layer, a secondary layer, a tertiarylayer, as well as other layers, with each layer being offset in heightrelative to the immediately preceding layer. The amount of offset may beany value commonly used by those having ordinary skill in the art, butmay depend on the expected ROP and durability of the primary layer aswell as the specific location and presence of the formation materialhaving grossly different mechanical and/or geologic properties (e.g.,chert, pyrite, nodules, etc.) in a given interval. The amount of offsetmay also depend on the anticipated overall ROP and durability of thedrill bit over the total interval to be drilled.

In accordance with embodiments of the invention the primary layercutting elements and the secondary (and possibly tertiary) cuttingelements occupy substantially different radial positions on theirrespective blades. This is illustrated in FIG. 4B, where one of theprimary layer cutting elements 406 a and a corresponding secondary layercutting element 408 a are shown. As can be seen, the primary layercutting element profile 406 a has a center C that lines up on one radialdisplacement X, whereas the corresponding secondary layer cuttingelement profile 408 a has a center C′ that lines up on a substantiallydifferent radial displacement X′. As a result, the corresponding cuttingelement in the secondary layer (and tertiary layer where applicable)cuts a substantially different swath (i.e., follows a substantiallydifferent path) through the subsurface formation from the primary layercutting element.

The width of the combined swath or cutting zone for the twocorresponding cutting elements, in accordance with embodiments of theinvention, spans a predetermined distance that, in some embodiments, isat least equal to the diameter of one of the cutting elements (i.e.,where one cutting element has a larger diameter than the other) and atmost equal to the diameter of one of the cutting elements plus apredetermined fraction of that diameter. This is illustrated FIGS.5A-5B, with FIG. 5A showing blades of different heights and FIG. 5Bshowing blades of the same height, but corresponding cutting elementsdeployed so as to have different heights. For either case, however, thecorresponding cutting elements in the different blades occupy differentradial positions.

In FIG. 5A, a drill bit 500 has blades 502 a and 502 b that are offsetat different heights, but with cutting elements 504 a-n and 506 a-n thatare set at the same height on their respective blades. Specifically, theblade in the lower half of the figure, blade 502 a, has a greater heightthan the blade 502 b in the upper half of the figure. Therefore, thecutting elements 504 a-n on the first blade 502 a form part of theprimary layer of cutting elements, while the cutting elements 506 a-n onthe second blade 502 b constitute part of the secondary layer of cuttingelements.

The cutting zone Z′, spanned by a given primary layer cutting elementand corresponding secondary layer cutting element, is at least equal tothe diameter D of one of these cutting elements and at most equal to thediameter D of one of the cutting elements plus a predetermined fractionof that diameter. That is, Z′=D+0.Y*D, where 0.Y is the predeterminedfraction In some embodiments, the diameter D used to define the cuttingzone Z′ is the diameter of the primary layer cutting element, while inother embodiments, the diameter D is that of the secondary layer cuttingelement. In still other embodiments, the diameter D used to define thecutting zone Z′ may be the diameter of whichever cutting element has thelarger diameter. And as for the predetermined fraction of the diameter,in some embodiments, this value may be about 1/10 to about ⅓, andpreferably about ⅕, of the diameter of whichever cutting element isused.

While the above arrangement has advantages, in some drillingapplications, it is desirable to have all the blades be at the sameheight for stability and durability purposes. FIG. 5B shows a drill bit508 with blades 510 a and 510 b that are the same height, but each bladehas cutting elements 512 a-n and 514 a-n that are deployed so as to havedifferent heights relative to each other. The technique used indeploying the cutting elements so as to have different heights may beany technique known to those having ordinary skill in the art. Forexample, the cutting elements may be recessed into the blades so as tohave different degrees of exposure, or each cutting element may beexposed by the same amount, but the surface of the blades may becontoured so that certain cutting elements are seated higher thanothers.

In the drill bit portion shown here, the non-shaded cutting elements 512a-n are deployed so as to have a greater height than the shaded cuttingelements 514 a-n. These non-shaded cutting elements 512 a-n accordinglyconstitute the primary layer of cutting elements, whereas the shadedcutting elements 514 a-n constitute the secondary layer of cuttingelements. Both shaded and non-shaded cutting elements 512 a-n and 514a-n may be intermixed across different blades 510 a and 510 b, asdepicted here, or cutting elements of different heights may be mountedon their own blades (similar to the implementation of FIG. 5A). Ineither case, the resulting cutting zone Z′ created by a given primarylayer cutting element and corresponding secondary layer cutting elementis at least equal to the diameter D of one of these cutting elements andat most equal to the diameter D of one of the cutting elements plus apredetermined fraction of that diameter (i.e, Z′=D+0.Y*D).

In the drill bits described thus far, multiple layers of cuttingelements have been achieved using a single row of cutting elements oneach blade. However, embodiments of the invention may also beimplemented using multiple rows of cutting elements on a single blade.Examples of such an arrangement are illustrated in FIGS. 6A-6B. In thefirst implementation FIG. 6A, a drill bit 600 has blades that are of thesame height, but at least one blade 602 has multiple (e.g., two) rows ofcutting elements 604 a and 604 b deployed thereon. The first row 604 a(lower half of the figure) has cutting elements 606 a-n that aredeployed so as to have a greater height than cutting elements 608 a-n inthe second row 604 b. These cutting elements 606 a-n, which may havedifferent shear lengths, therefore constitute part of the primary layerof cutting elements, whereas the cutting elements 608 a-n in the secondrow 604 b, which may also have different shear lengths, constitute partof the secondary layer.

The implementation of FIG. 6B is similar to the implementation of FIG.6A insofar as the drill bit 610 has blades that are of the same height,with at least one blade 612 having multiple (e.g., two) rows of cuttingelements 614 a and 614 b. However, instead of the same cutting elementheight, each row 614 a and 614 b contains a mix of cutting elementheights, with non-shaded cutting elements 616 a-n deployed so as to havea greater height than shaded cutting elements 618 a-n.

In both of the above implementations, the primary layer cutting elements606 a-n & 616 a-n and the corresponding secondary layer cutting elements608 a-n & 618 a-n occupy substantially different radial positions. As aresult, they cut substantially different swaths in the subsurfaceformation. The width of the swath or cutting zone, indicated again asZ′, is at least equal to the diameter D of one of these cutting elementsand at most equal to the diameter D of one of the cutting elements(e.g., the primary cutting element) plus a predetermined fraction ofthat diameter, or Z′=D+0.Y*D.

Referring back to the drill bit profile in FIGS. 4A-4B, in addition tooccupying substantially different radial positions, the primary layercutting elements 406 a-n have a shear length SL that is substantiallyequal to the shear length SL′ of the secondary layer cutting elements408 a-n. This may be desirable in some drilling applications, but asexplained above, the shear length affects a drill bit's stability anddurability, as well as its ability to effectively drill much longersections of different types of formation material. Therefore, in someembodiments, by endowing the primary and secondary layers of cuttingelements with substantially different shear lengths SL and SL′, therange of formation material that may be effectively drilled issignificantly increased.

FIGS. 7A-7B illustrate a segment of a drill bit profile 700 for anotherdrill bit having primary and secondary cutting element layers 702 and704 where the primary layer cutting elements 706 a-n and the secondarylayer cutting elements 708 a-n have substantially different shearlengths SL and SL′. Referring to FIG. 7A, the drill bit profile segment700 is similar to the drill bit profile segment 400 (see FIGS. 4A-4B)insofar as it represents a drill bit that may be achieved by deployingthe cutting elements 706 a-n and 708 a-n on blades of different heightsor maintaining the blades at the same height, then deploying the cuttingelements 706 a-n and 708 a-n so as to have different heights on one ormore blades. In addition, the primary layer cutting elements 706 a-n andthe secondary cutting elements 708 a-n (and possibly tertiary layercutting elements) also occupy substantially different radial positionson their respective blades. This is illustrated in FIG. 7B, where acenter C of one of the primary cutting elements 706 a lines up on aradial displacement X, and a center C′ of the corresponding secondarylayer cutting element 708 a lines up on a substantially different radialdisplacement X′.

Unlike the drill bit profile segment 400, however, the drill bit profilesegment 700 represents a drill bit where the primary layer cuttingelements 706 a-n have a shear length SL that is different from the shearlength SL′ of the secondary layer cutting elements 708 a-n. In thespecific embodiment of FIG. 7A, the primary layer shear length SL issmaller than the secondary layer shear length SL′. In other embodiments,however, the primary layer shear length SL may be larger than thesecondary layer shear length SL′, depending on the particular subsurfaceformation to be drilled or application challenges presented. Inembodiments where the primary layer cutting elements and/or secondarylayer cutting elements (and/or tertiary layer cutting elements) havemore than one shear lengths, the average shear length of the secondary(or tertiary) layer cutting elements is substantially different (e.g.,larger or smaller) from the average shear length of the primary layercutting elements. As a result of the different shear lengths and radialpositions, the cutting elements in the different layers cut independentswaths in the formation being drilled, and will therefore have differentcharacteristics in terms of durability, stability and ROP.

The width of the swath, or cutting zone, cut by a given primary layercutting element and a corresponding secondary (or tertiary) layercutting element in FIGS. 7A-7B spans a predetermined distance Z′ that,in some embodiments, is at least equal to the diameter D of one of thesecutting elements and at most equal to the diameter D of one of thecutting elements plus a predetermined fraction of that diameter (i.e.,Z′=D+0.Y*D). Note that with different shear lengths SL, the swath orcutting zone Z′ will vary with radial displacement (i.e., it is not afixed value). As mentioned above, in some embodiments, the diameter maybe the primary layer cutting element's diameter or it may be thesecondary layer cutting element's diameter, and the predeterminedfraction may be about 1/10 to about ⅓ percent of that diameter. In otherembodiments, the diameter used to define the cutting zone Z′ may be thediameter of whichever cutting element has the larger diameter.

In some embodiments, the sizes and shapes, and hence the diameters, ofthe primary and secondary layer cutting elements are substantially thesame. FIGS. 8A-8B illustrate an embodiment of the invention where thesizes of the primary and secondary layer cutting elements aresubstantially different. Referring to FIG. 8A, the embodiment shown hereis similar to the embodiment shown in FIGS. 4A-4B in that the drill bitprofile segment 800 represents a drill bit having primary and secondarycutting element layers 802 and 804. As before, the different layers 802and 804 may be achieved by deploying the cutting elements 806 a-n and808 a-n on blades of different heights or maintaining the blades at thesame height, then deploying the cutting elements 806 a-n and 808 a-n soas to have different heights on one or more blades. Like the embodimentshown in FIGS. 4A-4B, the primary and secondary layers have cuttingelements in substantially different radial positions (see FIG. 8B), andthus have independent and different bottom hole coverages and cutdifferent swaths in the formation being drilled. In some instances, thedifferent cutting elements belonging to the primary and secondary layersmay also have substantially different shear lengths SL and SL′ (oraverage shear lengths as applicable), similar to the embodiment of FIGS.7A-7B.

Unlike the embodiments shown in FIGS. 4A-4B and 7A-7B, the cuttingelements 808 a-n deployed on the secondary layer 804 in FIGS. 8A-8B havea substantially different size from the cutting elements 806 a-ndeployed on the primary layer 802. In the specific embodiment shown, thesecondary layer cutting elements 808 a-n have a diameter D′ that islarger than the diameter D of the primary layer cutting elements 806a-n. It is of course possible for the secondary layer cutting elements808 a-n to have a diameter D′ that is smaller than the diameter D of theprimary layer cutting elements 806 a-n, depending on the particularsequence of subsurface formation materials to be drilled in an interval.In embodiments where there are several sizes of cutting elementsdeployed on the primary and secondary layers, the average size of thesecondary layer cutting elements 808 a-n may be substantially different(e.g., larger or smaller) from the average size of the primary layercutting elements 806 a-n. Note again that with different shear lengthsSL, the swath or cutting zone Z′ will vary with radial displacement(i.e., it is not a fixed value).

While the drill bits discussed thus far have primary layer and secondarylayer cutting elements that are of substantially the same shape, namely,a round shape, other shapes may also be used. Examples of other shapesthat may be used include elliptical shapes, egg shapes, pear shapes, andteardrop shapes (hereinafter, collectively referred to as oval shapes),as well as other common and customized shapes known to those havingordinary skill in the art. In some cases, even non-circular shapes maybe used where at least a portion of the shape is flat (e.g.,semicircular, diamond, rectangular, etc). Moreover, embodiments of theinvention also provide a drill bit where the primary layer or primarycutting element tip profile and the cutting elements, and the secondarylayer or secondary cutting element tip profile and the cutting elements,have substantially different shapes and geometries.

FIGS. 9A-9B illustrate an embodiment of the invention where the primarylayer cutting elements and the secondary layer cutting elements havesubstantially different shapes. Referring to FIG. 9A, the embodimentshown here is similar to the previous embodiments insofar as the drillbit profile segment 900 represents a drill bit having primary andsecondary cutting element layers 902 and 904. As before, the differentlayers 902 and 904 may be achieved by deploying the cutting elements 906a-n and 908 a-n on blades of different heights or maintaining the bladesat the same height, then deploying the cutting elements 906 a-n and 908a-n so as to have different heights on one or more blades. Like theprevious embodiments, the primary and secondary layers have cuttingelements in substantially different radial positions (see FIG. 9B), andthus have independent and different bottom hole coverages and cutdifferent swaths in the formation being drilled. In some embodiments,the different cutting elements belonging to the primary and secondarylayers may also have substantially different shear lengths SL and SL′(or average shear lengths as applicable), similar to the embodiment ofFIGS. 7A-7B.

Unlike the embodiments shown in the previous figures, the cuttingelements 908 a-n deployed on the secondary layer 904 in FIGS. 9A-9B havea substantially different shape from the cutting elements 906 a-ndeployed on the primary layer 902. The difference in shape results insubstantially different axial volumes Av for the primary and secondarylayers 902 and 904. The term “axial volume,” as understood by thosehaving ordinary skill in the art, refers to the distance from the centerof the cutting element face to the cutting tip. In the specificembodiment shown, the secondary layer cutting elements 908 a-n have anoval shape, whereas the primary layer cutting elements 906 a-n areround. The oval cutting elements provide the secondary layer 904 with anaxial volume Av′ that is greater than the axial volume Av of the roundcutting elements of the primary layer 902.

The substantially different axial volumes affect the durability of thecutting elements in hard and abrasive formations. A larger axial volumeincreases the ability of the cutting element to withstand higherrotational speeds during the drilling process than a smaller axialvolume due to the substantially higher diamond content. For this reason,oval cutting elements are known to be highly effective in abrasiveformations or lithologies, such as sandstone and siltstone, from anaxial volume perspective. In addition, oval shaped cutting elements aremore effective at pre-fracturing of brittle formations, a characteristicthat improves ROP in carbonate bearing formations. Round cuttingelements, on the other hand, are more effective for shearing non-brittleformations or lithologies, such as shale, sandstones and siltstone.

By deploying oval cutting elements on the secondary layer 904 and roundcutting elements on the primary layer 902, embodiments of the inventioncombine the advantages of both round and oval cutting elements. Asimilar benefit may be obtained by deploying the round cutting elementson the secondary layer 904 and the oval cutting elements on the primarylayer 902. Alternatively, oval shaped cutting elements may be deployedon both the primary and secondary layers 902 and 904, but ofsubstantially different types. For example, elliptical shaped cuttingelements may be deployed on the primary layer 902 while teardrop cuttingelements may be deployed on the secondary layer 904, and so on.

In addition to substantially different shapes, in some embodiment, thecutting elements of the primary layer and the cutting elements of thesecondary layer may have substantially different back rake angles. Theterm “back rake angle,” as understood by those having ordinary skill inthe art, refers to the angle formed between a line parallel to thecutting element face and a vertical line drawn through the center of thecutting element. Such back rake ankles control how aggressively thecutting element engages the subsurface formation. In general, a smallerback rake angle increases cutting element aggressiveness (i.e., highROP), but leaves the cutting element vulnerable to impact breakage. Onthe other hand, a larger back rake angle decreases cutting elementaggressiveness (i.e., low ROP), but gives the cutting element longerlife.

FIGS. 10A-10C illustrate an embodiment of the invention where theprimary layer cutting elements and the secondary layer cutting elementshave substantially different back rake ankles. Referring to FIG. 10A,the embodiment shown here is similar to the embodiments shown in theprevious figures in that the drill bit profile segment 1000 represents adrill bit having primary and secondary cutting element layers 1002 and1004. As before, the different layers 1002 and 1004 may be achieved bydeploying the cutting elements 1006 a-n and 1008 a-n on blades ofdifferent heights or maintaining the blades at the same height, thendeploying the cutting elements 1006 a-n and 1008 a-n so as to havedifferent heights on one or more blades. Like the previous embodiments,the primary and secondary layers have cutting elements in substantiallydifferent radial positions (see FIG. 10B), and thus have independent anddifferent bottom hole coverages and cut different swaths in theformation being drilled. In some embodiments, the different cuttingelements belonging to the primary and secondary layers may also havesubstantially different shear lengths SL and SL′ (or average shearlengths as applicable), similar to the embodiment of FIGS. 7A-7B.

Unlike the previous embodiments, the cutting elements 1008 a-n deployedon the secondary layer 1004 here have a substantially different backrake angle from the cutting elements 1006 a-n deployed on the primarylayer 1002. FIG. 10C shows a side view of one of the cutting elements1010 deployed on the primary layer 1002 and one of the cutting elements1012 deployed on the secondary layer 1004. As can be seen, the secondarylayer cutting element 1012 has a back rake angle A′ that issubstantially different from the back rake angle A of the primary layercutting element 1010 relative to a subsurface formation 1014. In thespecific embodiment shown, the back rake angle A′ of the secondary layercutting elements 1012 is larger (e.g., 30°) than the back rake angle A(e.g., 20°) of the primary layer cutting elements 1010. This substantialdifference in back rake angle results in the secondary layer cuttingelement 1012 having decreased cutting element aggressiveness, but higherimpact resistance. In other embodiments, it is possible to have the backrake angle A′ of the secondary layer cutting element 1012 be smallerthan the back rake angle A of the primary layer cutting element 1010,depending on the levels of hardness and/or abrasiveness of the formationsequences to be drilled.

Although the embodiments described thus far have focused on thedifferent radial positions and shear lengths (or average shear lengthsas applicable), in some embodiments, it may be desirable to provide adrill bit where the primary layer cutting elements and the secondarylayer cutting elements having different shear lengths. An example ofsuch an embodiment is illustrated in FIGS. 11A-11B, where the drill bitprofile segment of 1100 is shown representing a drill bit having primaryand secondary cutting element layers 1102 and 1104. As before, thedifferent layers 1102 and 1104 may be achieved by deploying the cuttingelements 1106 a-n and 1108 a-n on blades of different heights ormaintaining the blades at the same height, then deploying the cuttingelements 1106 a-n and 1108 a-n so as to have different heights on one ormore blades.

Unlike the previous embodiments, the primary and secondary layers 1102and 1104 have cutting elements 1106 a-n and 1108 a-n mounted insubstantially identical radial positions (see FIG. 11B). Nevertheless,in accordance with embodiments of the invention, the cutting elementsbelonging to the primary and secondary layers may still havesubstantially different shear lengths S L and SL′ (or average shearlengths as applicable). In some embodiments, the different shear lengthsSL and SL′ (or average shear lengths as applicable) may be achieved bydeploying cutting elements having different shapes (e.g., round, oval,etc.) in the primary layer 1102 versus the secondary layer 1104, or viceversa. In other embodiments, although not expressly shown, the differentshear lengths SL and SL′ (or average shear lengths as applicable) may beachieved by deploying cutting elements having different sizes (e.g., 16mm, 19 mm, etc.) in the primary layer 1102 versus the secondary layer1104, or vice versa. Thus, although they share common radial positions,the cutting elements 1106 a-n and 1108 a-n of the primary and secondarylayers 1102 and 1104 still provide independent and different bottom holecoverages and cut different swaths in the formation being drilled.

In some embodiments, based on the specifics of an application as well asthe formation types to be drilled, the primary layer cutting elementsand the secondary layer cutting elements may have substantiallydifferent properties in terms of abrasion and impact resistance. Forexample, either the primary layer cutting elements or the secondarylayer cutting elements may be made more abrasion resistant (i.e., have afiner diamond grain), or both the primary layer cutting elements andsecondary layer cutting elements may have improved abrasion resistance.In a similar manner, the primary layer cutting elements may be made moreimpact-resistant than the secondary layer cutting elements, or viceversa, or both the primary layer and secondary layer cutting elementsmay have improved impact resistance.

In other embodiments, either the primary layer cutting elements or thesecondary layer cutting elements may be treated to remove catalyzingmaterial (e.g., cobalt), a process commonly referred to as “leaching.”As is well known in the art, leaching or removal of catalyzing materialfrom cutting elements can improve their thermally stability, thusallowing them to withstand much higher drilling temperatures beforefailing. Improved thermal stability drastically reduces the wearinitiation process of the cutting elements. This process may be used tofurther enhance the performance properties of the primary layer cuttingelements or the secondary layer cutting elements, as described herein.Techniques for removal of catalyzing material from cutting elements aregenerally known and may be found, for example, in U.S. Pat. No.8,544,408 entitled “High Volume Density Polycrystalline Diamond withWorking Surfaces Depleted of Catalyzing Material,” which is incorporatedherein by reference.

It should be noted that regardless of the diamond material types (e.g.,fine grain or coarse grain diamond materials) that may be used for theprimary layer and/or secondary layer cutting elements, or the leachingor catalyzing material depletion processes employed, all advantages,principles and teachings herein discussed for the present inventionremain valid and fully applicable to these various embodiments.

In operation, the cutting elements in the primary layer of the drill bitinitially bear most of the load during drilling of a specific dominantformation type (e.g., sandstone, shale, siltstone, etc.). As the cuttingelements in the primary layer wears and/or deteriorates due to formationhardness and/or abrasiveness, the cutting elements in the secondary andsubsequent layers define a new bit, having independent bottom holecoverage that cut different swaths in the formation, and also havedifferent and unique ROP, durability and stability characteristics.Based on the specific layout of a drill bit according to embodiments ofthe invention, but mainly due to the substantially different radialpositions of the cutting elements in the different layers and/orsubstantially different shear lengths (SL) of the different layers, suchdrill bits are adapted to effectively drill in chert, pyrite and ornodules due to the controlled and specifically staged durability and ROPcharacteristics of the drill bit of the invention. In such instances,the cutting elements in the primary layer fail, but in do doing so,expose the cutting elements in the secondary layer (and possiblytertiary layer, and so forth), which are then able to re-establish thedrill bit's ROP and durability characteristics, thus enabling the drillbit to continue drilling for longer periods of time at an effective ROP.In other words, because the secondary layer cutting elements haveindependent bottom hole coverage and may be customized with asubstantially different shear length, size, shape, thermal stability,abrasion resistance, and/or impact resistance according to embodimentsof the invention, the drill bit is able to continue drilling at aneconomical ROP through the subsequent formation type, eventuallyreentering the dominant formation type or a different formation that isdevoid of chert, pyrite or nodules.

While the present invention has been described with reference to one ormore particular embodiments, those skilled in the art will recognizethat many changes may be made thereto without departing from the scopeof the invention Accordingly, each of the foregoing embodiments andobvious variations thereof is contemplated as falling within the scopeof the claimed invention, as is set forth in the following claims.

1. A drill bit, comprising: a drill bit body; blades formed on saiddrill bit body, said blades having a plurality of cutting elementpositions radially located thereon; and cutting elements deployed onsaid blades, said cutting elements forming a primary layer of cuttingelements and a secondary layer of cutting elements, said primary layerof cutting elements having a different height relative to said drill bitbody from said secondary layer of cutting elements; wherein at least oneprimary layer cutting element and a corresponding secondary layercutting element occupy substantially different cutting element positionson said blades such that their cutting element profiles overlap, said atleast one primary layer cutting element and said corresponding secondarylayer cutting element together defining a cutting zone equal to adiameter of one of said at least one primary layer cutting element andsaid corresponding secondary layer cutting element plus a predeterminedfraction of said diameter.
 2. The drill bit according to claim 1,wherein said predetermined fraction is from approximately 1/10 toapproximately ⅓.
 3. The drill bit according to claim 1, wherein saidpredetermined fraction is approximately ⅕.
 4. The drill bit according toclaim 1, wherein said diameter is a diameter of said at least oneprimary layer cutting element.
 5. The drill bit according to claim 1,wherein said diameter is a diameter of said corresponding secondarylayer cutting element.
 6. The drill bit according to claim 1, whereinsaid cutting elements in said primary layer and said cutting elements insaid secondary layer have substantially different shear lengths.
 7. Thedrill bit according to claim 6, wherein said shear lengths are averageshear lengths derived from multiple shear lengths for said cuttingelements in said primary layer and said cutting elements in saidsecondary layer.
 8. The drill bit according to claim 1, wherein saidcutting elements in said primary layer and said cutting elements in saidsecondary layer have substantially different sizes.
 9. The drill bitaccording to claim 1, wherein said cutting elements in said primarylayer and said cutting elements in said secondary layer havesubstantially different shape.
 10. The drill bit according to claim 9,wherein said substantially different shapes result in said cuttingelements in said primary layer and said cutting elements in saidsecondary layer having substantially different axial volumes.
 11. Thedrill bit according to claim 1, wherein said cutting elements in saidprimary layer and said cutting elements in said secondary layer havesubstantially different abrasion resistances.
 12. The drill bitaccording to claim 1, wherein said cutting elements in said primarylayer and said cutting elements in said secondary layer havesubstantially different impact resistances.
 13. The drill bit accordingto claim 1, wherein said cutting elements in said primary layer and saidcutting elements in said secondary layer have substantially differentthermal stabilities.
 14. The drill bit according to claim 1, whereinsaid cutting elements in said primary layer and said cutting elements insaid secondary layer have substantially different back rake angles. 15.The drill bit according to claim 14, wherein one or more cuttingelements in said primary layer have a larger back rake angle than one ormore cutting elements in said secondary layer.
 16. The drill bitaccording to claim 14, wherein one or more cutting elements in saidprimary layer have a smaller back rake angle than one or more cuttingelements in said secondary layer.
 17. The drill bit according to claim1, wherein said cutting elements form a tertiary layer of cuttingelements and said primary layer of cutting elements have a differentheight relative to said drill bit body from said tertiary layer ofcutting elements, and wherein at least one primary layer cutting elementand a corresponding tertiary layer cutting element occupy substantiallydifferent cutting element positions on said blades such that theircutting element profiles overlap, said at least one primary layercutting element and said corresponding tertiary layer cutting elementtogether defining a cutting zone equal to a diameter of one of said atleast one primary layer cutting element and said corresponding tertiarylayer cutting element plus a predetermined fraction of said diameter.18. A method of assembling a drill bit, comprising: providing a drillbit body having blades formed thereon said blades having a plurality ofcutting element positions radially located thereon; and deployingcutting elements on said blades, said cutting elements forming a primarylayer of cutting elements and a secondary layer of cutting elements,said primary layer of cutting elements having a different heightrelative to said drill bit body from said secondary layer of cuttingelements; wherein at least one primary layer cutting element and acorresponding secondary layer cutting element occupy substantiallydifferent cutting element positions on said blades such that theircutting element profiles overlap, said at least one primary layercutting element and said corresponding secondary layer cutting elementtogether defining a cutting zone equal to a diameter of one of said atleast one primary layer cutting element and said corresponding secondarylayer cutting element plus a predetermined fraction of said diameter.19. The method according to claim 18, wherein said cutting elements ofsaid primary layer and said cutting elements of said secondary layer aremounted on blades having substantially different heights relative tosaid drill bit body.
 20. The method according to claim 18, wherein saidcutting elements of said primary layer and said cutting elements of saidsecondary layer are mounted on blades having substantially identicalheights relative to said drill bit body.
 21. The method according toclaim 20, wherein said cutting elements of said primary layer and saidcutting elements of said secondary layer are mounted on separate blades.22. The method according to claim 20, wherein one or more cuttingelements of said primary layer and one or more cutting elements of saidsecondary layer are mounted on a single blade.
 23. The method accordingto claim 20, wherein said one or more cutting elements of said primarylayer and said one or more cutting elements of said secondary layer aremounted on said single blade in multiple rows.
 24. The method accordingto claim 23, wherein at least one of said rows contains a combination ofcutting elements from said primary layer and cutting elements from saidsecondary layer.
 25. The method according to claim 23, wherein at leastone of said rows contains only cutting elements from said primary layeror only cutting elements from said secondary layer.
 26. The methodaccording to claim 18, wherein said primary layer of cutting elementsestablishes approximately 100% bottom hole coverage.
 27. A drill bitbody, comprising: blades formed on said drill bit body, and cuttingelement positions formed on said blades, said cutting element positionsradially located thereon such that when cutting elements are deployed onsaid blades, said cutting elements form a primary layer of cuttingelements and a secondary layer of cutting elements, said primary layerof cutting elements having a different height relative to said drill bitbody from said secondary layer of cutting elements; wherein at least oneprimary layer cutting element and a corresponding secondary layercutting element occupy substantially different cutting element positionson said blades such that their cutting element profiles overlap whensaid cutting elements are deployed on said blades, said at least oneprimary layer cutting element and said corresponding secondary layercutting element together defining a cutting zone equal to a diameter ofone of said at least one primary layer cutting element and saidcorresponding secondary layer cutting element plus a predeterminedfraction of said diameter.